1. Field of the Invention
The present invention relates to millimeter wave semiconductor devices used in a range of equal to or higher than the millimeter wave range.
2. Description of the Background Art
With the recent improvement and enhancement of information processing devices in speed, and image processing devices in resolution, there has been noted high-speed, large-capacity personal communications at radio frequencies such as a millimeter wave band having frequencies of 30 to 300 GHz, a centimetric-wave band or a quasi-millimeter wave band having frequencies close to 30 to 300 GHz.
For such communications it is necessary to make the most use of characteristics of radio waves as well as create a package adapted for radio frequencies which can be small in size and produced at reduced cost and which also require only a short period of time for development.
In general, radio-frequency (RF) packages tend to require sealing in view of the following three points: reduction of influences of unnecessary electromagnetic waves, maintenance of airtightness, and mechanical sealing. RF packages are mechanically sealed for the same reasons as general semiconductor packages. Sealing RF packages to maintain their airtightness is necessary because RF semiconductor chips in general tend to affect RF characteristics when humidity, temperature and the like vary.
To reduce influences of unnecessary electromagnetic waves, a factor which would not be considered for RF packages for relatively low frequencies such as mobile phones, Personal Handy-Phone System (PHS) is crucial in designing RF packages for millimeter wave frequencies and frequencies close thereto.
More specifically, in a millimeter wave-band range, a wavelength is 1 to 10 millimeters long in the atmosphere, and an effective wavelength would be approximately 100 microns to several millimeters when the dielectric constant of a material forming a package is taken into consideration. Since this length corresponds to a scale for a size of a RF semiconductor chip, a package or a RF circuit, its three-dimensional geometry and material properties such as dielectric constant, dielectric loss, significantly affect RF characteristics of the RF package. Thus the three-dimensional configuration internal to the package is an important factor in designing it.
FIG. 6 shows an exemplary configuration of a conventional RF package. Such RF packages are disclosed, for example, in Japanese Patent Laying-Open Nos. 08-018001 and 10-079623.
In this RF package, a RF semiconductor chip 30 with a RF circuit 32 formed on a semiconductor substrate 31 is fixed on an upper surface of a wiring substrate 10 via conductive paste 61. Chip 30 is fixed faceup, i.e., a surface thereof opposite to the surface bearing RF chip circuit 32 faces wiring substrate 10. Chip RF circuit 32 is connected via a wire 41 to a RF substrate circuit 12 of wiring substrate 10. The space in which RF semiconductor chip 30 is provided on wiring substrate 10 is sealed by a metal cap 57 for shielding electromagnetic waves.
FIG. 7 shows another exemplary configuration of a conventional RF package. It differs from the FIG. 6 configuration in that RF semiconductor chip 30 is connected to RF substrate circuit 12 of wiring substrate 10 via a bump 40 such that a surface bearing RF chip circuit 32 faces wiring substrate 10, i. e., facedown.
The FIG. 6 example, however, does not give any consideration to how the size of the space sealed by the cap affects electromagnetic-wave resonance and the like. Consequently, a large number of waveguide modes are created in the space at an frequency band applied and unnecessary electromagnetic waves leaking from RF chip circuit 32, RF substrate circuit 12, wire 14 and other components cause a large number of resonance in the space, resulting in significantly degraded RF characteristics.
Particularly, wire 41 can be a major cause of unnecessary electromagnetic waves, since it is difficult for wire 41 to achieve RF-matching due to its high inductance. Furthermore, with RF semiconductor chip 30 mounted faceup, the opposing cap's conductor can affect RF chip circuit 32, wire 41 and other components and change their transmission characteristics and the like.
In general the FIG. 7 conventional example generates less unnecessary electromagnetic waves than the FIG. 6 conventional example, since RF semiconductor chip 30 is mounted facedown via bump 40. However, as well as the FIG. 6 conventional example, this conventional example also fails to give consideration as to how the size of the space sealed by the cap affects electromagnetic-wave resonance and the like and a large number of waveguide modes will tend to be created in the space at a frequency band applied. Consequently, unnecessary electromagnetic waves leaking from the RF chip circuit, the RF substrate circuit and other components cause a large number of resonance in the space, resulting in significantly degraded RF characteristics.
The generation of waveguide modes provides a transmission characteristic for small loss and high energy. Thus even if a device with unsealed desirable RF characteristics has been obtained, the RF characteristics will significantly change when the device is sealed. In some cases, no RF characteristics may be obtained, and electromagnetic waves propagated through the space can affect and cause active elements to oscillate and emit heat so that the RF semiconductor chip may be destroyed. Even if it is not significantly degraded, it is necessary to design the RF circuit so that the changed characteristics after the device is sealed are desired characteristics. However, waveguide modes change depending on the arrangement of the RF semiconductor chip and chip parts for matching in the sealed space, the design of the RF substrate circuit, and the like. Accordingly, reviews and modifications are often required in designing a device and this extends the period for development of the device and increases the cost for designing the device.